“A Dialogue Concerning the Two Chief World Systems” by Galileo Galilei

A Gallup poll of 6 July 1999 of more than a thousand adults determined that only about four out of five Americans believe that the Earth orbits the Sun. Fully one in five believes that the Sun orbits the Earth. [1] It’s interesting that four in five Americans believe something that seems obviously and demonstrably false. The Earth doesn’t feel like its moving underfoot, and every day we see the Sun rise and set. We believe the moon orbits the Earth, and it sure looks like the behavior of the Sun and moon is pretty similar. Why do four out of five Americans believe that, contrary to our intuition, the Earth orbits the Sun?

I’ve been taking an informal survey of my friends, with just two questions. The first is whether the Earth or the Sun is the center of the solar system. [2] After I assure them it’s not a trick question, everyone answers the Sun. We’re beating Gallup’s averages. The second question confuses people: “How do you know?” The most common answer I’ve gotten is, “Duh, everyone knows that”. Some people know that Copernicus and Galileo got in trouble with the Church for saying the Sun was the center of the solar system. Others vaguely remember a science or history teacher telling them about “heliocentrism”. Fair enough.

When I rephrased the question to ask how human beings originally figured that the Earth orbits the Sun, my friends gave me a quizzical look. I was no better off than them; I knew that Galileo and Copernicus had gotten everyone to switch from the geocentric view to the heliocentric. I didn’t know how or why. How did those astronomers convince everyone to abandon more than a thousand years of careful science and replace it with a new framework? What arguments and data struck the telling blows for heliocentrism?

The Retrogradation of Mars

The work that I turned to to understand the arguments for heliocentrism is Galileo’s delightful Dialogue Concerning the Two Chief World Systems. It is a surprising book. The first surprise it delivers is its style and structure. Galileo imitates a Socratic dialogue. Like all good literature, conflict drives the plot of the Dialogue. In particular, the conflict between Ptolemy and Aristotle on the one hand, and Copernicus on the other, motivates Galileo. His protagonist, Salviati, argues for the Copernican world system. The canny, initially neutral Sagredo drives the conversation forward. The wryly named Simplicio attempts to prop up the Ptolemaic system. Simplicio’s bumbling affords Galileo an opportunity to clearly articulate his ideas about physics and the motion of the stars. Of Aristotle, Salviati says that “[w]e need guides in forests and in unknown lands, but on plains and in open places only the blind need guides. It is better for such people to stay at home, but anyone with eyes in his head and his wits about him could serve as a guide for them. In saying this I do not mean that a person should not listen to Aristotle; indeed, I applaud the reading and careful study of his works, and I reproach only those who give themselves up as slaves to him in such a way as to subscribe blindly to everything he says and take it as inviolable.” [3] The arguments about the motion of the heavens make the work science; the metaphors, plot, dialogue, and humor make it literature.

The second surprise in the Dialogue is Galileo’s outstanding ear for argument. He refutes Aristotle’s arguments bluntly and often amusingly. Aristotle claims that a ball of iron will fall one hundred cubits in the time a block of wood falls ten cubit. This is preposterously false, and Sagredo doubts whether Aristotle ever actually tried it out: “I greatly doubt that Aristotle ever tested by experiment whether it be true that two stones, one weighing ten times as much as the other, if allowed to fall, at the same instant, from a height of, say, 100 cubits, would so differ in speed that when the heavier had reached the ground, the other would not have fallen more than 10 cubits.” Simplicio leaps to the defense of his preposterously wrong philosopher by pointing out that “[Aristotle’s] language would seem to indicate that he had tried the experiment, because he says: ‘we see the heavier’; now the word ‘see’ shows he had made the experiment.” Galileo’s characters are forthright in dismissing this tomfoolery. They describe experimental procedures that we can repeat today that confirm Galileo’s ideas and to refute Aristotle’s.

Epicycles and Deferents

Claudius Ptolemy offers Galileo a foil with more scientific substance. [3] Ptolemy’s exposition of the geocentric world system in the unreadably complex Almagest is a surprisingly empirical and scientific work. Ptolemy’s system of deferents and epicycles is a compelling, mathematically precise, vision of the world. To refute it, Galileo works through a detailed analysis of the parallax of various celestial bodies and their distances from the Earth. The argument is technical and subtle. Writing before the great tectonic shift from Euclid to Descartes, Galileo’s mathematical reasoning tends towards geometry as well. Here he argues with Ptolemy on the latter’s home turf. A good edition that reproduces the figures is essential, as the characters lean on them as much as the reader. When a character references a table of astronomical data, Galileo’s language leads one to vividly imagine the characters sitting around a table passing papers to one another as they eat and drink.

The third surprise nestled in the Dialogue is that it’s deeply unsatisfying. It’d be tempting to say that, having read Galileo’s dialogue, I can answer the question about how we know the Earth goes around the Sun. The truth, though, is that I’ve just replaced one authority with another. I haven’t observed the phases of Venus, and I haven’t observed the parallax of the fixed stars (much less of the planets) that would let me work out the celestial mechanics on my own. Even so, reading Galileo’s argument is valuable. When I trust his authority, it’s an authority that I understand a little better than before. It’s an authority that’s grounded in reason and argument that I can wrestle with and assess critically. But if I want to add to the conversation myself, take a stand along with Ptolemy and Galileo, I need to telescope-up and start looking at the stars.

Too often we treat science as an Athena, sprung full grown and clothed from the head of Zeus. We trust our predecessors when they say that the Earth orbits the Sun without understanding how they came to believe that. We belittle people who disagree without understanding the arguments that make us correct. It turns out that the story of scientific discovery is always messy, usually fascinating, often wildly contingent, and occasionally well told. Whenever we can grapple with not only the results of scientific inquiry, but also the story behind it, we should. In his preface to the Dialogue, Einstein writes that, “[t]he leitmotif which I recognize in Galileo’s work is the passionate fight against any kind of dogma based on authority.” Galileo would rue his fate; like Aristotle before him, he’s become an authority, chalky and dead. We can rescue him from that cruel fate by listening carefully to the scientific conversation and the adding our voices to the tumultous banter.